The present invention relates to a process for producing a polyester composition, and more particularly, to an industrially useful process for producing a polyester composition to which an electrostatic pinning method usually adopted for molding amorphous sheets can be suitably applied.
Polyester films, typically polyethylene terephthalate films have been used in various application fields as base materials for, e.g., information recording media, capacitor films, wrapping films, films for printing plates, electrical insulating films, photographic films or the like, because of their excellent mechanical properties, electrical properties, chemical resistance or dimensional stability.
In general, polyester has been melt-extruded, quenched and solidified to form an amorphous sheet. Further, the amorphous sheet has been subjected to an orientation process to form a stretched film. In the production of such polyester films, edge portions of extruded polyester sheets have a larger thickness than that of the remaining portion thereof due to so-called neck-in phenomenon, and the edge portions having a larger thickness is used as a clamping clearance for clips. Therefore, the edge portions of the polyester films are cut or trimmed off as edge films. The edge films are pulverized, melt-extruded into pellets for reuse thereof.
The above-mentioned amorphous sheets have been generally formed by an electrostatic pinning method. The electrostatic pinning method comprises extruding a molten sheet over a rotary cooling drum and electrostatically charging the molten sheet by applying a D.C. voltage of about 5 to about 10 kV to an electrode arranged above the upper surface of the molten sheet perpendicularly to the delivery direction of the molten sheet, thereby enhancing the adhesion between the molten sheet and the rotary cooling drum (Japanese Patent Publication (KOKOKU) No. 37-6124(1962)).
In such an electrostatic pinning method, when the rotation speed of the rotary cooling drum is increased in order to improve the productivity, the adhesion force between the molten sheet and the rotary cooling drum is decreased, so that so-called constrained air bubbles of a crater shape are formed on the surface of the obtained sheet. The constrained air bubbles are likely to be formed as the specific resistance of raw polyester upon melting becomes increased. In order to reduce the specific resistance, there have been proposed various methods for incorporating a metal compound into the raw polyester.
As an example of these methods, in Japanese Patent Application Laid-open (KOKAI) No. 57-18534(1982), there have been proposed a method of adding a metal compound to polyester after the polymerization thereof. In this method, a metal salt of aliphatic monocarboxylic acid or dicarboxylic acid is added in an amount of 0.01 to 1% by weight based on the weight of polyester. In addition, from the standpoints of uniform blending and workability of the metal salt, a master batch containing a high concentration of the metal salt is produced and then mixed with polyester. However, in such a master batch process, when the raw polyester is melt-extruded into chips, there arises a problem that the molecular weight of polyester is inevitably decreased due to the high concentration of the metal salt.
In order to eliminate the above-mentioned problem, in Japanese Patent Publication (KOKOKU) No. 4-64328(1992), there has been proposed a method of controlling a melting temperature of the metal salt-added polyester to a specific range. However, there is a disadvantage that this method is applicable only to polyesters having a low melting point such as isophthalic acid copolyesters or polybutylene terephthalate, and not applicable to other polyesters.
It is an object of the present invention to provide a process for producing a polyester composition to which an electrostatic pinning method usually adopted for the shaping of amorphous sheets can be suitably applied, by adding a specific amount of a metal salt to polyester with a high accuracy.
It is another object of the present invention to provide an industrially useful process for producing a polyester composition to which an electrostatic pinning method usually adopted for forming amorphous sheets, can be suitably applied, especially by using recovered or reclaimed polyester as a raw material.
As a result of the present inventors"" various studies for accomplishing the aims, it has been found that:
(1) by using a specific vented extruder as described in Japanese Patent Application Laid-open (KOKAI) No. 6-91635(1994) which have been used only in limited cases where a slurry containing particles (mixture of solid and liquid) is mixed with polyester, and by limiting an amount of the metal salt added to polyester to a predetermined range, the conventional problems such as decrease in molecular weight of polyester due to the metal salt can be avoided, thereby readily accomplishing the above-mentioned objects;
(2) by using the vented extruder and feeding the polyester and the metal salt into the vented extruder through a common feed opening, it is possible to add the metal salt in an accurate amount;
(3) by using a regenerating apparatus comprising a pulverizer and an extruder both connected with each other, and by limiting an amount of the metal salt added to recovered or reclaimed polyester to a specific range, the problems such as decrease in molecular weight of the polyester due to the metal salt can be solved; and
(4) as the extruder used in the regenerating apparatus, there can be suitably used those having a vent port and a powder feed port.
The present invention has been attained on the basis of the finding.
In a first aspect of the present invention, there is provided a process for producing a polyester composition comprising polyester and a metal salt of aliphatic carboxylic acid, which process comprises providing a vented extruder; and feeding the metal salt together with polyester as a raw material having a specific resistance of not less than 1xc3x97107 xcexa9xc2x7cm upon melting, to a vented extruder, in such an amount that the concentration of metal atom based on the polyester composition is 0.5 to 1,000 ppm.
In a second aspect of the present invention, there is provided a process for producing a polyester composition comprising polyester and a metal salt of aliphatic carboxylic acid, which process comprises providing a regenerating apparatus comprising a pulverizer and a single screw extruder connected to the pulverizer; feeding recovered polyester having a specific resistance of not less than 1xc3x97107xcexa9xc2x7cm upon melting, to the pulverizer; pulverizing the recovered polyester in the pulverizer; and adding the metal salt to the pulverized recovered polyester in such an amount that the concentration of metal atom based on the polyester composition is 0.5 to 1,000 ppm, before the pulverized recovered polyester reaches the single screw extruder.
In a third aspect of the present invention, there is provided a process for producing a polyester composition comprising polyester and a metal salt of aliphatic carboxylic acid, which process comprises providing a regenerating apparatus comprising a pulverizer and a single screw extruder connected to the pulverizer, the single screw extruder having a vent port and a powder feed port arranged sequentially along the extruding direction thereof; feeding recovered polyester having a specific resistance of not less than 1xc3x97107 xcexa9xc2x7cm upon melting, to the pulverizer; pulverizing the recovered polyester in the pulverizer; and feeding the metal salt through the powder feed port into the single screw extruder to add the metal salt to the pulverized recovered polyester in such an amount that the concentration of metal atom based on the polyester composition is 0.5 to 1,000 ppm.